1. Field of the Invention
The present invention relates to a heat-developable lightsensitive element and more particularly to a heat-developable light-sensitive element having reduced thermal fogging (or undesired darkening in the unexposed areas generated during thermal development) and having improved light stability after development.
2. Description of the Prior Art
The most widely used photographic process is a process in which silver halides are employed since such a photographic process has superior photographic properties such as sensitivity and gradation to other photographic processes such as an electrophotographic process and a diazo process. There are, however, disadvantages in the silver halide photographic process. For example, the silver halide light-sensitive element must be developed in an aqueous developing solution after exposure and the thus developed light-sensitive element must be further subjected to several aqueous solution processings such as stopping, fixing and stabilization in order to prevent the thus-formed image from discoloring or fading and to prevent the undeveloped white areas in the image (background) from darkening. These solution processings are time-consuming and troublesome, and the chemicals employed in these processings are hazardous in handling and stain the worker's body and clothes and the processing room. The chemicals may also produce water pollution if the solutions are discharged without treatment.
Therefore, a light-sensitive element which utilizes high speed silver halides which can be processed in a dry manner without using processing solutions and which can be moveover capable of forming a stable image with minimized discoloration in the background areas thereof under normal room illumination has been desired.
To invent such elements various efforts have been made. For example, German Pat. Nos. 1,123,203 and 1,174,157 disclose that heat development of a silver halide light-sensitive element is possible by incorporating therein a 3-pyrazolidone-type developing agent. German Pat. No. 1,175,075 discloses that the heatdevelopability of the element can be promoted by incorporating therein a material capable of forming water, and German Pat. No. 1,003,578 discloses the additional incorporation therein of a fixing agent for the silver halide. According to the abovedescribed techniques, however, the silver halide itself remaining in the element after the dry processing would never be completely stabilized against light by a dry process, that is, the former three patents do not describe a dry fixing step, and in the lightsensitive element described in the last patent it can be easily appreciated that the copresence of a developing agent (a reductant) and a fixing agent during storage would give rise to an undesirable reaction which would make the element not very practical.
At present the most successful light-sensitive element capable of forming a photographic image by a dry processing method is a heat-developable light-sensitive element employing a composition comprising a silver salt of an organic acid, a small amount of a light-sensitive silver halide and a reducing agent, e.g., as described in U.S. Pat. Nos. 3,152,904 and 3,457,075. In this light-sensitive system the silver halide, which remains in the element after development, is not stabilized against light and is allowed to be discolored by light. Nevertheless the element provides results as if it were stabilized because the silver halide is used in a slight amount and most of the silver source is a white or light-colored organic silver salt which is stable and hardly blackened by light. Thus, even if a small amount of silver halide is discolored by light, the slight discoloring does not give rise to any difficulties visually since the element, on the whole, still appears white or only slightly colored. The light-sensitive element is stable at normal temperature, but when it is image-wise exposed and heated to a temperature, usually, above about 80.degree. C, preferably above 100.degree. C, the organic silver salt oxidizing agent and the reducing agent which are present in the light-sensitive element undergo oxidation-reduction reactions due to the catalytic action of the exposed silver halide present in the vicinity thereof to form silver. By this reaction, the exposed areas of the light-sensitive layer are rapidly blackened so that a contrast is formed between the exposed areas and the unexposed areas (background), that is, an image is formed.
In such a heat-developable light-sensitive element, a significant problem remains to be solved in that undesirable darkening in the unexposed or background areas (called heat fogging) tends to be produced upon heat development. This heat fogging relatively reduces the photographic densities in the image areas corresponding to exposed areas. Methods that have been used in order to eliminate this disadvantage include a method employing mercuric ions such as those described in Japanese Pat. Publication No. 11113/72. Mercury compounds are, however, highly poisonous and there is the danger that mercury may be evaporated during heat development. Further, secondary pollution may occur if used paper supports thereof are regenerated.
In spite of extensive attempts for reducing heat fogging, it is still quite difficult to inhibit heat fogging without impairing the photographic properties, which is well known to those skilled in the art. In general, the sensitivity is reduced by the addition of a heat-fogging inhibitor.